Between 1988 and 2008, the number of landfills in the US fell by seventy-seven percent (77%) based on a Municipal Solid Waste Report issued by the United Stated Environment Protection Agency (EPA). The price paid to landfills to take in waste (“tipping fee”), has increased by twenty-three percent (23%) between 2002 and 2008, creating a need to convert these types of waste feedstocks into higher value usages. To help reduce the volume of waste put into landfills, incineration has been used, but is not a viable long term option due to its negative environmental impacts. On the other hand, gasification, a process that converts carbonaceous materials or products into heat and power has also been used (with special attention to biomass products selection) in order to reduce the volume of waste feedstocks while simultaneously generating useable heat and power.
Conventional biomass gasification systems typically suffer from several issues that have currently limited their usage. Because of these limitations, restrictions on feed quality, size, moisture content, etc. must be imposed. Further, the current gasification strategies provide insufficient control over the key gasification steps and reactions, thereby yielding a non-optimized “producer gas” (PG) output stream which is low in calorific content, high in tar, and promotes incomplete char burnout. These problems can be traced, in many situations, to the crude feedstock injection and mixing strategies and non-optimal gasifier flow field conditions. The low calorific value PG thus generated is typically associated with less than a 70% gasification conversion efficiency, defined as the rate of energy output to input. The construction and costs associated with remedying these shortcomings can also be prohibitive. Remediation typically requires stringent feedstock control and/or costly preprocessing of the feedstock. Ash slagging and material handling further can cause additional complications and typically demand “pre-treated” (dried or compacted/densified) feedstocks. The costs associated with such refined biomass feedstocks are substantially greater than the lower cost and more readily available feedstocks, thereby adversely effecting system economics.
Therefore, to remedy these traditional technical and economic shortcomings, there is a need for a gasification system that produces an improved producer gas output stream from a variety of ‘low grade” or waste biomass materials, while minimizing tar production, energy input, and char carryover.